1. Technical Field of the Invention
The present invention relates to a gas cooling type vacuum heat treating furnace and a cooling gas direction switching device therefor.
2. Description of the Related Art
A vacuum heat treating furnace is the one in which inert gas or the like is refilled after depressurization therein in order to carry out heat-treatment of an article. Since the vacuum heat treating furnace may completely remove moisture or the like sticking to the interior of the furnace and to the treated article after heating, by depressurizing again the furnace after evaporation of the moisture or the like, and refilling the inert gas or the like thereinto, there may be exhibited such an merit that heat-treatment may be made without coloring by moisture (the so-called bright heat-treatment).
Further, a gas cooling type vacuum heat treating furnace may exhibit various merits such as capability of performing bright heat-treatment, causing no decarbonization, carburization and less deformation, and effecting a satisfactory working environment. However, a primary stage gas cooling type vacuum heat treating furnace was of a depressurizing and cooling type, and accordingly, its cooling speed was not sufficiently high so as to be disadvantageous. Thus, in order to increase the cooling speed, a high speed circulation gas cooling type one has been materialized.
Referring to FIG. 1 which shows a configuration of a high speed circulation gas cooling furnace disclosed in a non-patent document 1, there are shown a heat-insulating member 50, a heater 51, an effective operation zone 52, a furnace body 53 with a water jacket, a heat-exchanger 54, a turbo-fan 55, a fan motor 56, a cooling door 57, a hearth 58, a gas distributor 59, a damper 60 for switching flowing directions (air flow passage) of cooling gas.
Further, “Method of Promoting Gas Circulation Cooling in a Vacuum Furnace” (patent document 1), discloses a vacuum furnace comprising a heating chamber 66 surrounded by heat insulation walls 67 within a gas-tight vacuum vessel 61, as shown in FIG. 2, a heater 72 located in the heating chamber, for heating, under vacuum, an article 64 to be heated, and a cooler 62 and a fan 63 which are provided in the vacuum vessel 61, for cooling unoxidized gas fed into the vacuum vessel by the cooler 62 and then circulating the unoxidized gas in the heating chamber 66 through openings 68, 69 formed in the surfaces of those of the heat insulation walls 67 of the heating chamber 66 which are opposed to each other, by rotating the fan 63 in order to cool the article 64 to be heated under forced gas circulation, wherein a heat-resistant cylindrical hood 65 which is diverged at least at one end thereof, is arranged so as to surround the circumference of the article 64 to be heated which is located in the heating chamber 66, with a suitable space therebetween while opposite ends of the cylindrical hood 68 are opposed respectively to the openings 68, 69 in order to circulate the unoxidized gas through the heating chamber 66. Further, there is shown, in FIG. 2, a damper 40 for switching the flowing directions of the cooling gas.
[Non-patent document 1]
“Vacuum Heat Treatment for Metal Material (2)” authored by Katsuhiro Yamazaki, Heat-Treatment Vol. 30 No. 2, April in 1990
[Patent document 1]
Japanese Patent Laid-Open NO. 5-230528
The high speed circulation gas cooling furnaces disclosed in the non-patent document 1 and the patent document 1 have raised the following problems since the heating and cooling have been carried out at one and the same position:    (1) The heater and the furnace body which are at a high temperature after completion of heating are both cooled simultaneously with cooling of the article to be heat-treated, and accordingly, high speed cooling for the article to be heat-treated cannot be made;    (2) The article to be heat-treated is surrounded by the heater and the furnace body, and accordingly, cooling gas cannot be uniformly fed thereto during cooling;    (3) Even though the gas cooling is made alternately upward and downward, there are no means for rectifying both upward gas and downward gas so as to flow in uniform directions at uniform speeds, and accordingly, it has been difficult to reduce distortion of an article to be heat-treated in its entirely.
Further, in the high speed circulation gas cooling furnaces as disclosed in the non-patent document 1 and the patent document 1 as stated above, upper and lower damper units are in general provided as a mechanism for switching between upward and downward directions of gas flow (passages). However, in the case of using the upper and lower damper units as a cooling gas direction change-over mechanism, there have been raised the following problems:    (4) The damper units cause considerable variation in load exerted by a pressure of gas flowing at a high speed, depending upon an open and a close position thereof. Thus, in the case of high pressure gas, it is difficult due to affection by the pressure of air to smoothly operate such a damper system.    (5) The damper units have an opening area which is not proportional to an opening angle. Thus, upon switching of a plurality of upper and lower drive devices, it is difficult to balances opening areas in order, resulting in a difference in opening area between a suction port and a discharge port and considerable variation in the difference, and accordingly, the quantity of cooling gas varies so as to cause stable gas cooling to be difficult.    (6) There are presented a plurality of upper and lower damper units, and accordingly, a plurality of drive devices therefor are required, resulting in a complicated configuration.    (7) The opening areas are limited by the upper and lower dampers so as to be small in comparison with the interior surface of the furnace.